Multi-step Synthetic Routes (Cambridge (CIE) A Level Chemistry)
Revision Note
Multi-Step Synthetic Routes
A large number of organic products are made from a few starting compounds using appropriate reagents and conditions
Knowing how organic functional groups are related to each other is key to the synthesis of a given molecule
The main functional groups you need to know are
Alkanes
Alkenes
Haloalkanes
Nitriles
Amines
Alcohols
Carbonyls (aldehydes & ketones)
Hydroxynitriles
Carboxylic acids
Esters
Acyl chlorides
Primary and secondary amides
Examiner Tips and Tricks
You also need to be able to identify the functional groups of these chemicals in structures that are given to you
Aliphatic Reaction Pathways
The key interconversions between functional groups are summarised here:
Aliphatic Reactions Table
Reactant | Product | Reagents | Reaction |
|---|---|---|---|
Alkene | Hydrogen halide | Electrophilic addition | - |
Alkene | Alcohol | Hydration | Steam + H2SO4 / heat |
Alkene | Alkane | Hydrogen + Ni catalyst / 150 °C | Electrophilic addition / hydrogenation |
Alcohol | Alkene | Al2O3 or conc. acid / heat | Elimination / dehydration |
Alcohol | Haloalkane | NaX + H2SO4 / heat under reflux | Nucleophilic substitution |
Haloalkane | Alcohol | NaOH (aq) / heat under reflux | Nucleophilic substitution |
Alkane | Haloalkane | Halogen / UV light | Free radical substitution |
Primary alcohol | Aldehyde | Oxidation | K2Cr2O7 / H2SO4 / Distillation |
Secondary alcohol | Ketone | Oxidation | Heat |
Primary alcohol | Carboxylic acid | Oxidation | Heat under reflux |
Aldehyde | Primary alcohol | NaBH4 / H2O | Reduction |
Ketone | Secondary alcohol | NaBH4 / H2O, NaCN | Reduction |
Haloalkane | Nitrile | Nucleophilic substitution |
|
Haloalkane | Amine | NH3 / ethanol | Nucleophilic substitution |
Nitrile | Carboxylic acid | H2O / HCl | Hydrolysis |
Aldehyde | Hydroxynitrile | NaCN / H+ | Nucleophilic addition |
Alcohol | Ester | Esterification | Carboxylic acid / H2SO4 |
Carboxylic acid | Ester | Alcohol / H2SO4 | Esterification |
Ester | NaOH(aq) | Alkaline hydrolysis | Carboxylate salt and alcohol |
Ester | Carboxylic acid | Dilute acid | Acid hydrolysis |
Carboxylic acid | Acyl chloride | SOCl2 | Chlorination |
Acyl chloride | Carboxylic acid | H2O | Hydrolysis |
Acyl chloride | Primary amide | NH3 | Nucleophilic addition elimination |
Acyl chloride | Secondary amide | Primary amine | Nucleophilic addition elimination |
Aromatic Reaction Pathways
The key aromatic reactions are summarised here:
Aromatic Reactions Table
Reactant | Product | Reagents | Reaction |
|---|---|---|---|
Benzene | Methylbenzene | CH3Cl / AlCl3 | Alkylation / Electrophilic substitution |
Benzene | Bromobenzene | Br2 / FeBr3 | Bromination / Electrophilic substitution |
Benzene | Chlorobenzene | Cl2 / AlCl3 | Chlorination / Electrophilic substitution |
Benzene | Nitrobenzene | HNO3 / H2SO4 | Nitration / Electrophilic substitution |
Nitrobenzene | Aminobenzene / phenylamine / aniline | Sn / HCl | Reduction |
Aminobenzene | 2,4,6-tribromoaminobenzene / 2,4,6-tribromoaniline | Bromine | Electrophilic substitution |
Benzene | Phenylethanone | CH3COCl / AlCl3 | Acylation / Electrophilic substitution |
Phenylethanone | 1-Phenylethanol | NaBH4 | Reduction |
Designing a Reaction Pathway
The given molecule is usually called the target molecule and chemists try to design a synthesis as efficiently as possible
Designing a reaction pathway starts by drawing the structures of the target molecule and the starting molecule
Determine if they have the same number of carbon atoms
If you need to lengthen the carbon chain you will need to put on a nitrile group by nucleophilic substitution
Work out all the compounds that can be made from the starting molecule and all the molecules that can be made into the target molecule
Match the groups they have in common and work out the reagents and conditions needed
Worked Example
Suggest how the following synthesis could be carried out:
Ethene to 1-aminopropane
Answer
Examiner Tips and Tricks
Sound knowledge of all of the different reactions is beneficial as the A-level course simply states that you should be able to design a multistage synthesis
Past papers generally go to four steps in a multistep reaction although there is no clear limit stated
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